This invention relates to an improved permanent magnet motor, and also to an improved air delivery system for respirators in which a permanent magnet translational motor provides the linear drive for the respirator piston.
In the past, permanent magnet motors generally have been used in servo systems to apply translational motion to objects whose linear position is to be controlled. A permanent magnet motor generally includes a moving coil mounted on a core of ferromagnetic material which, in turn, is placed in a magnetic field produced by a permanent magnet. Electric current passing through the coil interacts with the magnetic field of the permanent magnet to produce a translational force on the coil parallel to the longitudinal axis of the core to move the coil, and a load attached to the coil, along a linear path. Periodic reversal of the current in the coil moves the coil in a reciprocating fashion to apply reciprocating longitudinal movement to the load.
One typical use of a permanent magnet translational motor is for driving a pen in a strip chart recorder. Such a motor is used in the strip chart recorder Models 7143A and 7143B manufactured by Hewlett Packard Corporation. This permanent magnet motor has a movable coil with a relatively wide air gap between the core and the permanent magnet. Because of the relatively wide air gap, a magnetic flux of relatively low intensity is produced across the gap. This develops a relatively small motor output force of about one pound. The pen drive motor also has a stroke length of about five to ten inches. A stroke of such length is attained because the magnetic field intensity and the current developed in the moving coil are both of such low magnitude that the coil can be moved axially through the five to ten inch stroke without the core being driven into saturation. Because the motor generates a small force, the moving coil may be coupled to the pen unsymmetrically.
Other permanent magnet translational motors have been designed for applications requiring a relatively large output force, say about 30 to 40 pounds. A typical motor used in such applications is the Model 14 computer memory head positioning motor manufactured by Information Magnetics Corporation. This motor includes a ferromagnetic housing with one open end, and a sliding coil mounted on a relatively short, cylindrical steel core inside the housing. The coil moves back and forth in a narrow air gap, which enables a relatively large flux to be developed across the gap. The current passing through the coil interacts with the large flux to produce an output force of about forty pounds at a stroke length of about two inches. The motor is not suitable for use in applications requiring a long stroke length in the neighborhood of about five to ten inches, because the relatively large field over the longer length drives the core into saturation, for motors with stroke lengths of more than two inches, unless a larger cross-sectional area of iron is used.